Single-molecule spectroscopy of conjugated molecules between nanoscale metallic structures
Final Report Abstract
In this project the optical properties of metal-molecule contacts were investigated using well-defined metallic nanostructures interacting with organic molecules. Firstly, individual lithographically fabricated gold nanostructures, i.e. gold bowtie nanoantennas, were used for the first time as substrates for surface-enhanced Raman scattering (SERS) under controlled electrochemical potential. SERS spectra of para-mercaptoaniline molecules chemisorbed to bowties show a surprising switching-on behavior of b2-symmetric vibrational modes of the molecule upon electrochemical cycling, accompanied by electrochemical roughening of the gold surface. In combination with theoretical considerations the initial adsorption geometry of the molecules was determined. Our findings underscore the necessity of well-characterized substrates for a meaningful analysis of surfaceenhanced Raman spectra and may also have implications for the development of SERS-based sensors operating in liquid environments. Secondly, ITO patterning processes have been optimized to improve patterning of this important, optically inert but conducting material. A minimum feature size of ~50nm was achieved for the first time. This opens new avenues of research in combining electrical transport and optical measurements of metallic nanostructures without perturbing effects originating from the nanometer size scale of the metallic object. Finally, single-molecule Raman signals have been observed from gold nanoparticle dumbbells linked by only one biphenylddithiol molecule. Evidence for the single-molecule nature of the Raman signal is based on ultralow concentration arguments and the observation of spectral diffusion. This geometry is believed to represent a well-defined universal Raman detection scheme for single molecules since it only uses the plasmonic properties of the metallic nanostructure but does not rely on the electronic properties of the molecule. Furthermore dumbbells can be trapped electrostatically between electrodes to perform simultaneous electrical and optical single-molecule experiments.
Publications
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Gold bowtie nanoantennas for surfaceenhanced Raman scattering under controlled electrochemical potential. Chem. Rhys. Lett, 2007, 446, 339-343
F. Jäckel, A.A. Kinkhabwala and W.E. Moerner
